The conventional blast furnace comprises a lower portion which includes the hearth section and bosh section and an upper portion which includes the stack, the receiving hopper, distributor, downcomer and bells. The stack or shaft section of the blast furnace is enclosed by a truncated cone-shaped shell with steep sides which are continuous from the top downward to the mantle girder. A concrete foundation is designed to support the loads from the furnace structure and its burden. The lower portion of the furnace is supported directly on the foundation. A plurality of columns surrounding the furnace are also supported on the foundation. A mantle girder mounted on top of the columns surrounds the furnace adjacent the bosh section and is connected to the furnace shell to carry the load of the furnace structure and burden from the upper portion of the furnace.
Prior to the advent of the high pressure top in the blast furnace, the accepted design for bosh steelwork consisted of bosh band construction which was connected throughout the bosh with steel hanger bars. This steelwork design was adequate since the hanger bars between the bands permitted the bosh bands to expand and contract independently without transmitting loads to the furnace columns, mantle, or tuyere jacket.
Increased furnace pressures made it necessary to change from the bosh band design to a solid bosh jacket in which holes were provided for installing in-wall cooling plates. The bosh jacket design adequately contained the brickwork construction which supported the in-wall cooling plates. However, the design did not contain the internal pressures and gases produced.
Consequently, with the development of carbon bosh technology, the solid jacket, spray-cooled bosh has evolved as an acceptable bosh design. This concept is currently in use on many operating blast furnaces.
The bosh jacket design has not been trouble-free, however. Blast furnace productivity, in many cases, has been increased significantly beyond design considerations. This has resulted in decreased effectiveness of much of the furnace cooling system. Marginal cooling in the lower stack, stop-and-go operations, and inadequate in-wall cooling plate technology contribute to premature bosh distress.
Bosh distress, which usually occurs in the mantle area, results from inadequate cooling, coupled with a high magnitude of shell restraint caused by the furnace mantle ring which is rigidly fixed to the bosh jacket. When the bosh jacket becomes overheated and expands, the mantle section does not expand equally. This differential expansion leads to high stresses in the bosh jacket and subsequent cracking through the bosh plate and inside mantle flange.
These bosh problems are associated with the mantle-to-bosh configuration.